One important use of lasers is in communications and information transfer, wherein one or more lasers transmit modulated light signals through one end of an optical fiber or cable. A receiver at the opposite end of the cable then converts the laser light into useful information. Laser intensity noise is one of the limiting factors in the transmission of analog or digital signals. Laser intensity noise reduces the ratio of signal-to-noise and increases bit error rates.
Relative intensity noise (RIN), describes the dynamic range of either the laser or the associated fiberoptic communications system. RIN can be thought of as a kind of inverse carrier-to-noise measurement, or as a measure of the spectral distribution of the noise content. More specifically RIN is defined as the ratio of the means square optical intensity noise to the square of the average optical power. Total system noise is the combination of laser intensity noise, thermal noise from the electronics and noise, thermal noise from the electronics and photonic shot noise. RIN is usually expressed in db per Hz of bandwidth.
Semiconductor laser sources, such as laser diodes and laser diode arrays, can be used alone or they can be used to pump another laser or laser system, such as a laser using Nd:YAG (i.e., neodymium doped yttrium aluminum garnet) as the lasant material. However used, a laser diode is activated or driven to produce laser light in response to the flow of DC electrical current from a power supply. When the RIN of the output of a diode laser system or a diode-pumped solid-state laser is observed, it is characterized by distinct or dominant peak or a spectral region of high RIN. This peak is the result of the phenomenon of relaxation oscillation. A. E. Siegman, Lasers, University Science Books, Mill Valley, Calif.; Chapter 25, 1986. Relaxation oscillations are small amplitude, quasi-sinusoidal exponentially damped oscillations. For diode pumped solid-state lasers (e.g., Nd:YAG), the spectral region or frequency band is typically between 10 KHz and 10 MHz. Relaxation oscillation is thought to be dependent on the fluorescence lifetime of the lasant material, cavity lifetime of the laser system, and cavity losses.
One means of reducing RIN in a diode-pumped solid-state laser is to use electronic feedback. Kane, "Intensity Noise in Diode-Pumped Single-Frequency Nd:YAG Lasers and its Control by Electric Feedback", IEEE Photonics Technology Letters, Vol 2, No. 4, April 1990. In particular, a photo-diode was used to sense the output of the laser system, and an amplifier was used to convert the output of the photo-diode into a phase shifted feedback signal which is added to the output of the DC Power Supply which is used to drive the system's laser diode. A positive phase shift, applied at all frequencies from the relaxation oscillation frequency up to the frequency where loop gain goes below unity, was needed to avoid instability. This phase lead was accomplished by designing the amplifier so that gain is rising as a function of frequency, as is the case of a differentiator.
The Kane feedback circuit does not take in consideration changes in the performance of the laser diode over the life of the laser diode. The relaxation oscillation frequency of a laser system changes over its life. Moreover, there is no teaching or suggestion as to how feedback could be used to cover a broad range of frequencies or how feedback could be tailored to the spectrum of the relative intensity noise. Thus, there is much room for improvement.